A contact lens is a thin lens made of an optically transparent material such as plastic or glass that is fitted over the cornea of the eye to correct vision defects. Various types of contact lenses exist that are designed to treat various types of vision defects such as myopia, hyperopia, presbyopia or astigmatism, and combinations of these defects. Contact lens types can be further divided into “rigid” contact lenses, which rest on the cornea of the eye, and “soft” contact lenses, which rest on the cornea and surrounding sclera of the eye.
Typical contact lenses have a central portion, which is the optical portion of the lens, and a peripheral portion, which is the carrier portion of the lens. The carrier portion typically contains a transition, or blending, zone where the optical portion and the carrier portion meet. The optical portion typically extends from the center of the lens outwardly to a distance of approximately 3.5 to 4 millimeters (mm) where the optical portion meets the carrier portion. This corresponds to a sagittal radius, r, that ranges from r=0.0 mm at the center of the lens to r≈3.5 or 4.0 mm at the boundary where the optical and carrier portions of the lens meet. The carrier portion of a typical contact lens starts where the optical portion ends (e.g., at r≈3.5 or 4.0 mm) and extends outwardly a radial distance from the lens center of r≈7.0. Thus, the typical contact lens has a total diameter of approximately 14.0 mm.
In typical contact lens designs, the optical portion of the lens provides optical power for vision correction. The carrier portion of the lens serves to stabilize the lens and fit the lens comfortably over the cornea and/or limbus of the eye, but normally is not designed to provide vision correction. It is generally accepted that central vision is more accurate than peripheral vision. The highest concentration of photoreceptors is in a small depression near the center of the retina known as the fovea centralis. The fovea centralis is about 0.2 mm in diameter, representing about 20 minutes of angle on either side of the visual axis of the eye. Acuity drops dramatically in the peripheral region of the retina such that at about 5 degrees off of the visual axis, the acuity has dropped to about ⅓ of the central value.
While contact lenses typically are not designed to provide optical control over peripheral vision, it has been suggested that the peripheral retina may have important effects on the emmetropization system that controls the growth of the eye. For example, it has been suggested that blur and defocus in the peripheral retina have an effect on the axial eye growth and play a role in the development of refractive errors such as myopia. Myopia is the medical term for nearsightedness. Myopia results from excessive growth of the eyeball along its longitudinal axis. Individuals with myopia see objects that are closer to the eye more clearly, while more distant objects appear blurred or fuzzy. These individuals are unable to see distant objects clearly without a correction lens. Because excessive axial growth of the eyeball typically continues throughout childhood and adolescence, the condition of nearsightedness usually worsens over time. Myopia has become one of the most prevalent vision problems. Furthermore, myopic individuals tend to be predisposed to a number of serious ocular disorders, such as retinal detachment or glaucoma, for example. Presumably, this is because of the anatomical distortions that exist in the enlarged myopic eye. Extreme cases of these disorders are among the leading causes of blindness.
It is generally accepted that myopia is caused by a combination of an individual's genetic factors and environmental factors. Multiple complex genetic factors are associated with the development of refractive error. Currently, no genetic treatment approach exists for preventing or slowing the progression of myopia. Researchers have proposed that accommodative lag at near vision provides hyperopic defocus stimulus that leads to excessive axial eye growth, and thus to the development of myopia. It has been proposed that the use of a lens that provides on-axis myopic defocus can remove the on-axis hyperopic defocus that leads to excessive eye growth. For example, researchers have shown that myopic children who wore progressive addition lenses (PALs) exhibited reduced myopia progression over three years as compared to an age-matched and refraction-matched population of children who wore single vision lenses over an equal time period. The PALs create on-axis myopic defocus. It is presumed that the on-axis myopic defocus provided by the PALs removes the on-axis hyperopic defocus created by the optics, resulting in a reduction in myopia progression.
It has also been proposed that peripheral hyperopic defocus may stimulate axial eye growth, thereby leading to the progression of myopia. The optical treatment system that has been proposed to counter this effect comprises a lens that is designed to remove hyperopic defocus by creating a myopic shift in refraction peripherally (i.e., off-axis), while providing no central (i.e., on-axis) effect. To performs these functions, the lens is provided with: (1) on-axis optics that are optimized for central refraction such that any central (on-axis) retinal defocus created by the optics of the eye is minimized to provide the best possible central visual acuity; and (2) off-axis optics that are tailored to provide peripheral (off-axis) myopic defocus that corrects for the peripheral (off-axis) hyperopic defocus. Therefore, this approach is intended to only remove peripheral (off-axis) hyperopic defocus created by the optics of the eye and is not intended to have any effect on central (on-axis) hyperopic defocus created by the optics of the eye.
While this approach may be suitable for individuals who are at relatively advanced stages of myopia, it may not suitable for individuals who are only slightly myopic or who are in early stages of myopia. In individuals who are only slightly myopic or who are in early stages of myopia, little or no peripheral hyperopia exists when considering refractive status for near vision (i.e., for close visual work). In these cases, the peripheral myopic defocus is excessive and can produce peripheral hyperopic stimulus, which may actually speed up the progression of myopia. Therefore, in such cases, using a lens that creates peripheral myopic defocus is not an adequate solution for preventing or slowing the progression of myopia.
Accordingly, a need exists for a lens design and method that are effective at preventing or slowing the progression of myopia.